Conventional economy brand caulking guns are generally manual trigger-operated devices incorporating a unidirectional gripping assembly which urges a piston rod forward to eject the compound from a cartridge. One such caulk gun is set forth in U.S. Pat. No. 4,081,112 issued to Chang. The Chang '112 caulking gun positions the trigger pivot and trigger drive grip engagement above the plunger shaft. This improvement increases the leverage obtainable by a hand operated trigger and allows robust delivery of the composition at a higher volume and flow rate than was previously possible in a hand-held caulking gun. This helps to deliver a variety of dense fluid compounds including urethane, vinyl, polyester, epoxy and other plastics or resins.
In addition to their density, many of these compounds have other properties such as fast setting times that make them difficult to remove if improperly applied. Consequently, in addition to being robust, the dispensing device must be capable of applying a clean and uniform bead of compound. In large part, the quality of the bead depends on the users ability to control the volumetric flow rate. Optimally, control should be maintained at all times to insure a uniform bead. However, complete control is difficult in light of the different densities of material to be applied, and the different conditions encountered during application. Control is often lacking when the user wishes to terminate or reduce the bead of caulk. Most prior art caulk guns maintain full pressure when the trigger is released, and the user is required to depress a release lever to terminate the bead. This introduces a short lag time between the decision to terminate or reduce the bead and depression of the release lever. Within this lag time of maintained pressure, an unwanted surplus of compound is extruded and a messy and uneven bead often results. It would be best if pressure could be relieved upon initial release of the trigger, thereby cutting off the flow of caulk. However, the plunger must quickly be locked in place to prevent rearward retraction. Otherwise, it will be difficult to continue dispensing of compound in a uniform bead.
There have been past efforts to achieve this momentary releasing action in the past. For example, U.S. Pat. No. 4,566,610 issued to Herb discloses a dual-cartridge dispensing device utilizing a pair of angled grips 16 (see FIG. 1) which engage a releasing member 19 upon full release of the trigger to thereby remove all driving force and free the plunger shafts. Unfortunately, the releasing mechanism of Herb '610 is adapted for that particular drive assembly, and the drive assembly is complex and expensive.
U.S. Pat. No. 4,461,407 to Finnegan discloses an automatic pressure relief mechanism for a caulk gun including an annular elastic ring 42 tightly encircling the plunger shaft 8. The ring 42 is held against the interior of housing 21 by a fixed retainer 200. When the trigger 130 is retracted, the plunger shaft 124 is urged forward through the ring 42. Since it is tight, the ring deforms and is partially pulled through the housing wall 21. When the trigger 130 is released, the deformed ring 42 tries to regain its shape, thereby pulling shaft 124 back a bit. This releases pressure, and a dripstop feature is introduced. Despite the advantage, the Finnegan '407 caulk gun has its drawbacks. Specifically, the attachment of the frictional ring 42 to the wall of housing 21 impedes the motion of the plunger shaft 124. The elastic ring 42 becomes the subject of deformation and wear. Moreover, the ring 42 and retainer 200 assembly is rather costly as it requires intricate metal forming (see FIG. 2), welding (column 4, lines 24-26), and a precision-fit elastic ring (see column 4, lines 63-68). These factors escalate the manufacturing costs.
U.S. Pat. No. 5,156,305 to Eyre discloses a drive assembly for a molded-plastic open frame caulk gun. In this open frame type (see FIG. 2), the plunger shaft is slidably carried by two sleeves 28, 30 formed in a molded plastic housing. A downwardly extending trigger 43 is pivoted to the housing and retractable against the handle 46. The trigger 43 includes an upper rivet 45 above the pivot point. The upper rivet 45 bears against a first gripping member 41 that encircles the plunger shaft, and a compression spring 42 rearwardly biases the first gripping member 41 toward the trigger 43. A release lever 51 encircles the plunger shaft and extends upwardly behind the housing to a pivot point. Like Finnegan '407, Eyre '305 also teaches the use of a rod-engaging O-ring bush 52 that relieves pressure upon release of the trigger. In contrast to Finnegan '407, Eyre '305 attaches the bush 52 to the release lever 51 to move therewith (see, also, column 4, lines 18). The plunger shaft frictionally passes through the elastic bush 52. In operation, the release lever 51 with integral bush 52 rides forward on the plunger shaft while the trigger 43 is retracted. When the trigger 43 is first released, the friction of the bush 52 catches the plunger shaft and the release lever 51 is carried backward. The release lever 51 eventually attains a critical angle and engages the plunger shaft to prevent further retraction. However, pressure is released in the meantime, and a dripstop feature is introduced. As did Finnegan '407, Eyre '305 leaves room for improvement. The bush 52 and housing 53 assembly is rather costly as it requires drilling of a hole through the release lever, insertion of the bush 52 in the housing 53, and insertion of the combination into the release lever hole. The additional parts and labor greatly increases manufacturing costs and assembly time.
It would be advantageous to provide a robust drive assembly at lower manufacturing costs.